Polypeptide as standard for proteome analysis

ABSTRACT

The present invention provides a polypeptide as standard for peptide analysis by mass spectrometry comprising at least 16 peptides selected from the group consisting of the peptides of SEQ ID NO: 1 to SEQ ID NO: 22 or variants thereof, together with an artificial protein comprising the polypeptide, a vector comprising a nucleic acid encoding the polypeptide, a kit for proteome analysis, a selection of peptides for calibration an devaluation of mass spectrometers and chromatographs for proteome analyses and uses thereof.

FIELD OF THE INVENTION

The present invention relates to the field of proteomics, in particular to the quantification of proteins and to polypeptide standards to optimise the separation of peptides by reversed-phase chromatography and their detection and fragmentation by mass spectrometry.

The present invention therefore provides a polypeptide as standard for peptide analysis by mass spectrometry which comprises at least 16 peptides, preferably 18 peptides, more preferably 20 peptides, most preferably 22 peptides selected from the group consisting of the peptides of SEQ ID NO: 1 to SEQ ID NO: 22 and/or functional variants thereof. The invention is further directed to an artificial protein comprising such a polypeptide and a vector comprising a nucleic acid encoding the polypeptide and/or the artificial protein. Additionally, the invention encompasses a kit for proteome analysis, a selection of peptides for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses and the use of the peptides according to the invention for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses.

BACKGROUND OF THE INVENTION

With the ever increasing number of published manuscripts reporting peptide characterisation by reversed-phase chromatography coupled with mass spectrometric analysis, there is a pressing need to precisely define the instrument conditions used for these analyses. At present, instrument calibration and optimisation is performed on a laboratory-by laboratory basis, with no two facilities using the same criteria for instrument set-up. Many laboratories using multiple mass spectrometers for analysis of the same sample also use different standards for calibration and optimisation of the different instruments. In addition, the chromatographic conditions like solvents, solid-phase and elution gradient used for separation of peptides by reverse-phase are seldom the same. This makes both intra- and inter-laboratory comparisons of proteomics data almost impossible to perform with any degree of consistency.

EP 1 736 480 A1, Beynon et al., 2005 and Pratt et al., 2006 describe a Qcon-CAT methodology for the construction of tryptic peptide sequences but do not disclose a single polypeptide standard for optimising separation of peptides by reversed-phase chromatography and their detection and fragmentation by mass spectrometry, in addition to maintaining reproducibility in proteomics experiments, which requires that instrument parameters be optimised and standardised according to defined criteria. No single standard currently exists which can be used to assess instrument performance in this manner.

Thus there is still an existing need for such a single polypeptide standard.

SUMMARY OF THE INVENTION

The present invention therefore provides a polypeptide as standard for peptide analysis by mass spectrometry which comprises at least 16 peptides, preferably 18 peptides, more preferably 20 peptides, most preferably 22 peptides selected from the group consisting of the peptides of SEQ ID NO: 1 to SEQ ID NO: 22 and/or functional variants thereof. The invention is further directed to an artificial protein comprising such a polypeptide and a vector comprising a nucleic acid encoding the polypeptide and/or the artificial protein. Additionally, the invention encompasses a kit for proteome analysis, a selection of peptides for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses and the use of the peptides according to the invention for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows design and implementation of the standard polypeptide QCAL.

FIG. 1A: Unique Q-peptides were chosen to assess peptide separation by reversed-phase chromatography and for optimisation and calibration of a range of mass spectrometers. The sequences of each of these peptides was concatenated in silico and used to generate a QconCAT construct as described in the section “Examples”. Listed are the monoisotopic masses of the

[M+H⁺ and [M+2H]²⁺ ions. Each peptide is present as single copy except ^(a)Q9 with 3 copies and ^(b)Q10 with 6 copies.

FIG. 1B: After expression in E. coli BL21 (DE3) cells and purification by affinity chromatography on a Ni-NTA column, QCAL was digested with trypsin and the peptides analysed by MALDI-ToF MS.

FIG. 1C: Digested QCAL was further analysed using an FT ICR MS.

FIG. 1D: Upper panel: A high-resolution FT ICR mass spectrum, distinguishing the doubly charged species of Q1 and Q7, a difference of 0.0182 Th is shown. Lower panel: Data was collected over a range of m/z 900-1500 which confirms that the resolution of this instrument is sufficient to readily detect peptide deamidation. In this case, deamidated Q9 is depicted.

FIG. 2 shows the analysis of QCAL by LC-MS/MS. Tryptic peptides from QCAL (500 fmol) were separated by reversed-phase chromatography using a PepMap™ C18 columns (5 μm, 0.075×150 mm, 100 Å) from LC Packings. Chromatography was performed at 300 nl/min with bound peptides being eluted over a 30 min gradient from 90% A (1% MeCN, 0.1% FA), 10% B (50% MeCN, 0.1% FA) to 30% A, 70% B, arranged in-line with a QToF micro (Waters).

FIG. 2A depicts the base peak chromatogram of eluted QCAL peptides.

FIG. 2B depicts the tandem MS spectra of Q10.

FIG. 2C depicts MS spectra for the section of the LC gradient (34.8-35.8 min) over which peptides Q8, Q9, Q10 eluted is also shown. The area under the curve for the first 5 isotope peaks was used to calculate peptide response.

FIG. 3 shows the effect of guanidination on MALDI-ToF MS of Q8 and Q11. Trypsinised QCAL was analysed by MALDI-ToF MS before and after guanidination as described in the section “Examples”. Depicted is m/z 1410-1470, clearly indicating preferential detection of the lysine-terminating Q11 peptide after guanidination (Q11*). Based on the change in isotope distribution post-guanidination, base catalysed deamidation of the peptides can also be observed as described in Song et al., 2001.

DETAILED DESCRIPTION OF THE INVENTION

This invention describes a tool, termed QCAL, designed specifically to optimise and define instrument conditions for peptide analysis by mass spectrometry in the absence or presence of upfront peptide separation by reversed-phase chromatography.

In a first aspect, the present invention is directed to a polypeptide as standard for peptide analysis by mass spectrometry which comprises at least 16 peptides, preferably 18 peptides, more preferably 20 peptides, most preferably 22 peptides selected from the group consisting of the peptides of SEQ ID NO: 1 to SEQ ID NO: 22 and/or functional variants thereof. The peptides of SEQ ID NO: 1 to SEQ ID NO: 22 are also termed Q1 to Q22.

The term “functional variant” or “functional variants” of a peptide sequence means that conservative amino acid substitutions, like acid (Asp and Glu) or basic amino acid (Asn and Gln) substitutions can be made on the present peptides. Also aromatic amino acids may be exchanged against each other.

In a preferred embodiment individual peptides of SEQ ID NO: 1 to SEQ ID NO: 22 can be present in more than one peptide copy.

In particular, the peptide of SEQ ID NO: 9 can be present in up to 5 copies, preferably up to 3 copies; most preferably in 3 copies and the peptide of SEQ ID NO: 10 can be present in up to 10 copies, preferably up to 6 copies, most preferably in 6 copies.

In a further aspect, the present invention provides an artificial protein comprising the aforementioned polypeptide.

An additional aspect of the invention is a vector comprising a nucleic acid encoding the aforementioned polypeptide and/or the artificial protein.

The invention further encompasses a kit for proteome analysis. The kit may comprise the aforementioned polypeptide as standard for peptide analysis by mass spectrometry, the aforementioned artificial protein and/or the aforementioned vector.

The invention is further directed to a selection of peptides for calibration and evaluation of mass spectrometers and chromatographs for proteome analysis. Therefore the use of an inventive protein or polypeptide for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses is also encompassed.

The artificial polypeptide, termed QCAL, was constructed using the QconCAT methodology and comprises 22 unique tryptic peptide sequences (SEQ ID NO: 1 to SEQ ID NO:22—FIG. 1A). The polypeptide is designed for calibration of a range of mass spectrometers typically used in peptide analysis.

In addition, the QCAL peptides were designed to facilitate the optimisation of instrument resolution, test the linearity of signal detection, as well as peptide separation by reversed-phase chromatography. Moreover, as these peptides are incorporated into an artificial protein, generation of these standards requires the end-user to validate sample preparation procedures such as tryptic digestion and desalting conditions. Characteristics are also incorporated within the design to assess peptide modification such as deamidation, methionine oxidation and modification of lysine residues, for example by guanidination.

Peptide mass fingerprinting (PMF) experiments are typically performed using a matrix-assisted laser-desorption ionisation (MALDI) time-of-flight (ToF) mass spectrometer (MS) for the identification of proteins following in-gel digestion with trypsin (Cottrell, 1994). The tryptic peptides that are generated under these conditions and subsequently used for database searching and protein identification are typically between m/z 500 and 3500. However, as calibration of MALDI-ToF instruments for peptide analysis is generally performed with a mixture of synthetic peptides, calibration is usually not performed lower than m/z 800-900, even though peptides in this region may enhance search algorithm scores and subsequent protein identification.

QCAL, was therefore designed to incorporate tryptic peptides with [M+H]³⁰ monoisotopic masses of m/z<500 and m/z>3000. Analysis of QCAL peptides by MALDI-ToF MS (FIG. 1B and Table 1 below) confirms that these peptides provide suitable scope for instrument calibration over the mass range required for PMF studies.

TABLE 1 Monoisotopic masses measured by MALDI-ToF MS of trypsin digested QCAL. Theoretical Measured Q-peptide Sequence [M + H]⁺ [M + H]⁺ Δppm SEQ ID NO: 1 VFDEFKPLVEEPQNLIR 2073.1021 2072.9913 −53.45 SEQ ID NO: 2 VFDEFKPLVKPEEPQNLIR 2298.2498 2298.1500 −43.44 SEQ ID NO: 3 VFDEFKPLVKPEEKPQNLIR 2426.3448 2426.2551 −36.97 SEQ ID NO: 4 VFDEFKPLVKPEEKPQNKPLIR 2651.4925 2651.4009 −34.55 SEQ ID NO: 5 VFKPDEFKPLVKPEEKPQNKPLIR 2876.6403 2876.5580 −28.59 SEQ ID NO: 6 VFKPDEFKPLVKPEEKPQNKPLIKPR 3101.7880 3101.7332 −17.66 SEQ ID NO: 7 VFDEFQPLVEEPQNLIR 2073.0657 N.D. N.D. SEQ ID NO: 8 GVNDNEEGFFSAR 1441.6348 1441.5444 −62.73 SEQ ID NO: 9 GGVNDNEEGFFSAR^(a) 1498.6563 1498.5707 −57.12 SEQ ID NO: 10 GGGVNDNEEGFFSAR^(b) 1555.6778 1555.6038 −47.54 SEQ ID NO: 11 GVNDNEEGFFSAK 1413.6287 1413.5381 −64.08 SEQ ID NO: 12 AVMDDFAAFVEK 1342.6354 1342.5354 −74.45 SEQ ID NO: 13 AVMMDDFAAFVEK 1473.6758 1473.6070 −46.71 SEQ ID NO: 14 AVMMMDDFAAFVEK 1604.7163 N.D. N.D. SEQ ID NO: 15 GLVK 416.2873 N.D. N.D. SEQ ID NO: 16 FVVPR 617.3776 617.3346 −69.57 SEQ ID NO: 17 ALELFR 748.4358 748.3755 −80.55 SEQ ID NO: 18 IGDYAGIK 836.4518 N.D. N.D. SEQ ID NO: 19 EALDFFAR 968.4842 N.D. N.D. SEQ ID NO: 20 YLGYLEQLLR 1267.7051 N.D. N.D. SEQ ID NO: 21 VLYPNDNFFEGK 1442.6957 N.D. N.D. SEQ ID NO: 22 LFTFHADICTLPDTEK 1850.8999 N.D. N.D. Listed for each Q-peptide is the monoisotopic mass of the singly charged peptide (theoretical and measured) and the ppm difference. ^(a)Q9 is present with a copy number of 3, ^(b)Q10 with a copy number of 6. N.D. - not detected.

Given the preferential detection of arginine-terminating tryptic peptides over their lysine-terminating counterparts by MALDI-ToF, PMF studies often benefit from being performed on tryptic peptides after conversion of lysine to homoarginine, thus improving peptide signal intensity (Brancia et al., 2000).

The almost identical Q-peptides 8 and 11 (FIG. 1A) were therefore specifically incorporated to test the efficiency of C-terminal homoarginine formation by lysine guanidination, with detection of Q11 only being possible after guanidination (FIG. 3).

High resolution mass spectrometers, such as the Fourier Transform Ion Cy-clotron Resonance (FT ICR) and the orbitrap, which allow the determination of analyte masses to high accuracy (sub ppm mass difference), are becoming increasingly popular in proteomics applications, primarily due to the reduction in false positive peptide identification. Calibration of these instruments within the standard window for proteomics applications can be achieved using QCAL either as an external calibrant (FIG. 1C) (where average mass accuracy sub 1 ppm was achieved, and Table 2) or as an internal calibrant.

TABLE 2 Masses measured by FT ICR MS of trypsin digested QCAL Theoretical Measured Theoretical Measured Q-peptide Sequence [M + H]⁺ [M + H]⁺ Δppm [M + 2H]²⁺ [M + 2H]²⁺ Δppm SEQ ID NO: 1 VFDEFKPLVEEPQNLIR 2073.1021 N.D. N.D. 1037.0550 1037.0548 −0.18 SEQ ID NO: 2 VFDEFKPLVKPEEPQNLIR 2298.2498 N.D. N.D. 1149.6289 1149.6279 −0.83 SEQ ID NO: 3 VFDEFKPLVKPEEKPQNLIR 2426.3448 N.D. N.D. 1213.6763 1213.6772 0.72 SEQ ID NO: 4 VFDEFKPLVKPEEKPQNKPLIR 2651.4925 N.D. N.D. 1326.2502 N.D. N.D. SEQ ID NO: 5 VFKPDEFKPLVKPEEKPQNKPLIR 2876.6403 N.D. N.D. 1438.8241 N.D. N.D. SEQ ID NO: 6 VFKPDEFKPLVKPEEKPQNKPLIKP 3101.7880 N.D. N.D. 1551.3979 N.D. N.D. SEQ ID NO: 7 VFDEFQPLVEEPQNLIR 2073.0657 N.D. N.D. 1037.0368 1037.0368 0.00 SEQ ID NO: 8 GVNDNEEGFFSAR 1441.6348 1441.6351 0.18 721.3214 721.3216 0.35 SEQ ID NO: 9 GGVNDNEEGFFSAR^(a) 1498.6563 1498.6585 1.47 749.8321 749.8312 −1.19 SEQ ID NO: 10 GGGVNDNEEGPFSAR^(b) 1555.6778 1555.6769 −0.55 778.3428 778.3421 −0.93 SEQ ID NO: 11 GVNDNEEGFFSAK 1413.6287 1413.6310 1.63 707.3183 707.3185 0.31 SEQ ID NO: 12 AVMDDFAAFVEK 1342.6354 1342.6350 −0.27 671.8216 671.8206 −1.50 SEQ ID NO: 13 AVMMDDFAAFVEK 1473.6758 1473.6778 1.33 737.3419 737.3418 −0.08 SEO ID NO: 14 AVMMMDDFAAFVEK 1604.7163 1604.7138 −1.58 802.8821 802.8819 −0.25 SEQ ID NO: 15 GLVK 416.2873 N.D. N.D. 208.6476 N.D. N.D. SEQ ID NO: 16 FVVPR 617.3776 617.3763 −2.02 309.1927 N.D. N.D. SEQ ID NO: 17 ALELFR 748.4358 748.4358 0.01 374.7218 N.D. N.D. SEQ ID NO: 18 IGDYAGIK 836.4518 N.D. N.D. 418.7299 N.D. N.D. SEQ ID NO: 19 EALDFFAR 968.4842 N.D. N.D. 484.7460 N.D. N.D. SEQ ID NO: 20 YLGYLEQLLR 1267.7051 N.D. N.D. 634.3565 N.D. N.D. SEQ ID NO: 21 VLYPNDNFFEGK 1442.6957 N.D. N.D. 721.8518 N.D. N.D. SEQ ID NO: 22 LFTFHADICTLPDTEK 1850.8999 N.D. N.D. 925.9539 N.D. N.D. Theoretical Measured Theoretical Measured Q-peptide Sequence [M + 3H]³⁺ [M + 3H]³⁺ Δppm [M + 4H]⁴⁺ [M + 4H]⁴⁺ Δppm SEQ ID NO: 1 VFDEFKPLVEEPQNLIR 691.7060 691.7043 −2.39 SEQ ID NO: 2 VFDEFKPLVKPEEPQNLIR 766.7552 766.7540 −1.55 575.3184 575.3183 −0.10 SEQ ID NO: 3 VFDEFKPLVKPEEKPQNLIR 809.4535 809.4518 −2.11 607.3421 607.3446 4.12 SEQ ID NO: 4 VFDEFKPLVKPEEKPQNKPLIR 884.5028 884.5013 −1.64 663.6290 663.6298 1.16 SEQ ID NO: 5 VFKPDEFKPLVKPEEKPQNKPLIR 959.5520 959.5529 0.94 719.9160 719.914 −2.72 SEQ ID NO: 6 VFKPDEFKPLVKPEEKPQNKPLIKP 1034.6012 1034.6025 1.22 776.2029 776.2035 0.77 SEQ ID NO: 7 VFDEFQPLVEEPQNLIR SEQ ID NO: 8 GVNDNEEGFFSAR SEQ ID NO: 9 GGVNDNEEGFFSAR^(a) SEQ ID NO: 10 GGGVNDNEEGPFSAR^(b) SEQ ID NO: 11 GVNDNEEGFFSAK SEQ ID NO: 12 AVMDDFAAFVEK SEQ ID NO: 13 AVMMDDFAAFVEK SEO ID NO: 14 AVMMMDDFAAFVEK SEQ ID NO: 15 GLVK SEQ ID NO: 16 FVVPR SEQ ID NO: 17 ALELFR SEQ ID NO: 18 IGDYAGIK SEQ ID NO: 19 EALDFFAR SEQ ID NO: 20 YLGYLEQLLR SEQ ID NO: 21 VLYPNDNFFEGK SEQ ID NO: 22 LFTFHADICTLPDTEK 617.6388 N.D. N.D. Listed for each Q-peptide is the monoisotopic mass of the singly, doubly, triply and quadruply charged peptide where appropriate (theoretical and measured) and the ppm difference. ^(a)Q9 is present with a copy number of 3, ^(b)Q10 with a copy number of 6. N.D. - not detected.

To test the resolving power of instruments such as these, QCAL was designed to incorporate peptides Q1 and Q7, with a lysine to glutamine substitution. This results in a difference of 0.0364 amu and distinguishing these two peptides requires instrument resolution in excess of 57,000, quite within the capabilities of both the mass spectrometers mentioned above. Data acquired on a 9.4T FT ICR instrument indicates that, as expected, these two peptides can be readily distinguished (FIG. 1D, top panel), with peak resolution in excess of 105,000 being achieved. Detection of both of these peptides can therefore be used as a benchmark for instrument resolution. In addition, deamidation of a number of tryptic peptides from QCAL was also observed, with detection of the deamidated form of Q9²⁺ (FIG. 1 D, bottom label) requiring an instrument resolution in excess of 94,000. Detection of these deamidated species can thus be used as an additional specification for standardising the performance of high resolution instruments.

Critical to the success of proteomics experiments and the characterisation of peptides within complex mixtures is their separation by reversed-phase chromatography prior to mass spectrometric analysis. QCAL was therefore designed to incorporate peptides with a range of hydrophobicities, thereby permitting evaluation of reversed-phase chromatographic conditions for peptide separation. Reversed-phase chromatography of the QCAL tryptic peptides shows that they elute between 5 and ˜35% acetonitrile (FIG. 2A and Table 3 below), the typical range over which most tryptic peptides elute from C₁₈ reversed-phase chromatographic media (Sun et al., 2004; Washburn, 2001).

TABLE 3 Hopps- Wood Elution Hydro- time Q-peptlde Sequence [M + H]⁺ phobicity (min) SEQ ID NO: 1 VFDEFKPLVEEPQNLIR 2073.1021 0.29 40.89 SEQ ID NO: 2 VFDEFKPLVKPEEPQNLIR 2298.2498 0.42 39.21 SEQ ID NO: 3 VFDEFKPLVKPEEKPQNLIR 2426.3448 0.55 36.01 SEQ ID NO: 4 VFDEFKPLVKPEEKPQNKPLIR 2651.4925 0.64 32.94 SEQ ID NO: 5 VFKPDEFKPLVKPEEKPQNKPLIR 2876.6403 0.71 31.15 SEQ ID NO: 6 VFKPDEFKPLVKPEEKPQNKPLIKPR 3101.7880 0.77 29.95 SEQ ID NO: 7 VFDEFQPLVEEPQNLIR 2073.0657 0.13 40.93 SEQ ID NO: 8 GVNDNEEGFFSAR 1441.6348 0.44 35.52 SEQ ID NO: 9 GGVNDNEEGFFSAR^(a) 1498.6563 0.35 35.18 SEQ ID NO: 10 GGGVNDNEEGFFSAR^(b) 1555.6778 0.38 35.07 SEQ ID NO: 11 GVNDNEEGFFSAK 1413.6287 0.44 34.64 SEQ ID NO: 12 AVMDDFAAFVEK 1342.6354 0.10 41.02 SEQ ID NO: 13 AVMMDDFAAFVEK 1473.6758 −0.01 41.13 SEQ ID NO: 14 AVMMMDDFAAFVEK 1604.7163 −0.10 42.23 SEQ ID NO: 15 GLVK 416.2873 −0.08 29.75 SEQ ID NO: 16 FVVPR 617.3776 −0.50 28.06 SEQ ID NO: 17 ALELFR 748.4358 −0.10 36.29 SEQ ID NO: 18 IGDYAGIK 836.4518 −0.05 28.11 SEQ ID NO: 19 EALDFFAR 968.4842 0.15 40.91 SEQ ID NO: 20 YLGYLEQLLR 1267.7051 −0.56 N.D. SEQ ID NO: 21 VLYPNDNFFEGK 1442.6957 −0.10 39.64 SEQ ID NO: 22 LFTFHADICTLPDTEK 1850.8999 −0.10 N.D. Listed for each Q-peptide is the Hopps-Wood hydrophobicity index, together with the elution time following reversed-phase chromatography. N.D. - not detected.

A significant number of mass spectrometry laboratories use the Glu-fibrinogen peptide (GVNDNEEGFFSAR—SEQ ID NO: 8) for testing instrument sensitivity and also for calibration of the ToF following fragmentation. However, this sometimes requires a different instrument set-up (for example, analyte infusion) than is used for peptide analysis by LC-MS. This peptide sequence was therefore incorporated into QCAL (Q8) to permit calibration post-fragmentation (FIG. 2B). Incorporation of Q8 also permits the testing of instrument sensitivity using the same configuration as is used for proteomics studies. This enables the end-user to optimise the position of the ionisation needle, thus maximising signal-to-noise detection for peptide analysis. The range of m/z and charge states of the peptides included in QCAL also permits optimisation of the rolling collision energy required to obtain high quality tandem MS spectra and thus the best possible peptide identification.

For quantification studies in particular, characterising the linearity of signal detection of the instrument is also critical. Multiple copies of two modified forms of the Glu-fibrinogen peptide, where one (Q9, three copies) or two (Q10, six copies) additional glycine residues have been added to the peptide amino-terminus, were thus included in QCAL (FIG. 1A). Analysis of these three peptides by LC-MS on a quadrupole-time of flight (Q-ToF) instrument demonstrates that the additional glycine residues negatively affected peptide detection, with each additional glycine residue reducing peptide detection by ˜15%. Instead of detecting a ratio of 1:3:6 for Q8:Q9:Q10, they were seen in a ratio of 1:2.6:3.9 (FIG. 2C). Similar changes in detection factors were also detected following LC-MS on a quadrupole ion trap (data not shown). However, MALDI-ToF analysis of these peptides (FIG. 1 B) demonstrated a ratio of 1.0:3.4:6.0, closer to the actual peptide representation, indicating less glycine-dependent changes in peptide detection.

The data of this invention demonstrates that a single standard, QCAL, can be used for calibration and parameter optimisation of a number of instruments widely used in proteomics studies. Furthermore, it is believed that it will be possible to use this standard for testing and comparison in the development of new techniques and instruments for peptide analysis. More significantly, this standard will enable the proteomics community to define in more detail the behaviour of the instruments used in large-scale studies, thus facilitating long-term reproducibility in proteomics projects.

Examples

In the following, relevant methods for QCAL construction are described.

1. QCAL Construction.

-   -   Q-peptides were designed to assess mass spectrometer calibration         and resolution, linearity of signal detection (by virtue of         multiple copies of Q-peptides 9 and 10), peptide separation by         reversed-phase chromatography and specific modifications         incorporated during sample preparation: deamidation,         modification of lysine, methionine and cysteine residues. The         peptide sequences were then randomly concatenated in silico and         used to direct the design of a gene, codon-optimised for         expression in E. coli. The predicted transcript was subsequently         analysed for RNA secondary structure that might diminish         expression. Additional peptide sequences were added to provide         an initiator methionine residue (MGALR—SEQ ID NO: 23), a His₆         sequence (ALVALVHHHHHH—SEQ ID NO: 24) for affinity purification         using Ni-NTA resin and a sequence for removal of the tag with         endoproteinases (ALVALVLVPRGSLEVLFQGPIEGRTENLYFQGDDDDK—SEQ ID         NO: 25). The gene was synthesised and cloned into the expression         vector pET21a.

2. Expression and Sample Preparation.

QCAL was expressed and purified as previously described (Pratt et al., 2006), diluted to 1 mg/ml in 50 mM ammonium bicarbonate and digested with trypsin (2% (w/w), O/N). Digested QCAL (1 nmol) was dried by vacuum centrifugation and guanidination of lysine residues was performed by addition of ammonium hydroxide (7M, 10 μl) and O-methylisourea (0.5 M in water, 5 μl). After overnight incubation, samples were dried as above and desalted using C18 ZipTips (Millipore, Watford, UK) prior to MALDI-ToF analysis.

3. Fourier Transform Ion Cyclotron Resonance (FT ICR) Mass Spectrometry.

-   -   Digested QCAL was desalted using a C₁₈ peptide trap (Michrom         Bioresources), dried by vacuum centrifugation and resuspended in         50% (v/v) acetonitrile, 0.1% (v/v) formic acid to 1 pmol/μl. The         masses of the eluted pepetides were analysed using a Bruker         Daltonics Apex III™ 9.4T FT ICR mass spectrometer (Billerica,         Mass.) and an electrospray source, following infusion. Data         acquisition was performed manually using the Bruker Xmass™         software, version 6.01 (Bruker Daltonics, Bremen, Germany). Mass         spectra were collected using 512 data points per scan, over a         mass range of 50-5000 m/z. High resolution data was collected         over a mass range of 650-1500 m/z.

4. Matrix-Assisted Laser-Desorption Ionisation-Time of Flight (MALDI-ToF) Mass Spectrometry.

MALDI-ToF MS analysis was performed, using a Voyager-DE™ STR (Applied Biosystems) with digested QCAL crystallised with a saturated solution of alpha-cyanocinnamic acid in 50% (v/v) acetonitrile, 0.1% trifluoroacetic acid. Detection was performed in reflector mode with delayed extraction at 200 nsec.

REFERENCES

-   -   1. Beynon, R. J., Doherty, M. K., Pratt, J. M. & Gaskell, S. J.         Multiplexed absolute quantification in proteomics using         artificial QCAT proteins of concatenated signature peptides. Nat         Methods 2, 587-9 (2005).     -   2. Brancia, F. L., Oliver, S. G. & Gaskell, S. J. Improved         matrix-assisted laser desorption/ionization mass spectrometric         analysis of tryptic hydrolysates of proteins following         guanidination of lysine-containing peptides. Rapid Commun Mass         Spectrom 14, 2070-3 (2000).     -   3. Cottrell, J. S. Protein identification by peptide mass         fingerprinting. Pept Res 7, 115-24 (1994).     -   4. Pratt, J. M. et al. Multiplexed absolute quantification for         proteomics using concatenated signature peptides encoded by         QconCAT genes. Nat Protocols 1, 1029-43 (2006).     -   5. Song, Y., Schowen, R. L., Borchardt, R. T. & Topp, E. M.         Effect of ‘pH’ on the rate of asparagine deamidation in         polymeric formulations: ‘pH’-rate profile. J Pharm Sci 90,         141-56 (2001).     -   6. Sun, W., Wu, S., Wang, X., Zheng, D. & Gao, Y. A systematical         analysis of tryptic peptide identification with reverse phase         liquid chromatography and electrospray ion trap mass         spectrometry. Genomics Proteomics Bioinformatics 2, 174-83         (2004).     -   7. Washburn, M. P., Wolters, D. & Yates, J. R., 3rd. Large-scale         analysis of the yeast proteome by multidimensional protein         identification technology. Nat Biotechnol 19, 242-7 (2001). 

1. A polypeptide as standard for peptide analysis by mass spectrometry comprising at least 16 peptides, preferably 18 peptides, more preferably 20 peptides, most preferably 22 peptides selected from the group consisting of the peptides of SEQ ID NO: 1 to SEQ ID NO: 22 and/or functional variants thereof.
 2. The polypeptide of claim 1, wherein individual peptides of SEQ ID NO: 1 to SEQ ID NO: 22 are present in more than one peptide copy.
 3. The polypeptide of claim 1 or 2, wherein the peptide of SEQ ID NO: 9 is present in up to 5 copies, preferably up to 3 copies, most preferably in 3 copies.
 4. The polypeptide of claim 1 or 2, wherein the peptide of SEQ ID NO: 10 is present in up to 10 copies, preferably up to 6 copies, most preferably in 6 copies.
 5. An artificial protein comprising a polypeptide of any of the preceding claims.
 6. A vector comprising a nucleic acid encoding the polypeptide of any of claims 1 to 4 and/or the artificial protein of claim
 5. 7. A kit for proteome analysis comprising a polypeptide as standard for peptide analysis by mass spectrometry of any of claims 1 to 4, an artificial protein of claim 5 and/or a vector of claim
 6. 8. A selection of peptides for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses.
 9. Use of a polypeptide of any of claims 1 to 4 for calibration and evaluation of mass spectrometers and chromatographs for proteome analyses. 